It is often convenient to remove aldehydes or vinyl esters from process streams. This is desirable because aldehydes or vinyl esters such as vinyl acetate are reactive materials and may interfere with subsequent utilization of the stream. For example, in the preparation of vinyl acetate/ethylene copolymers, a stream of waste gases is produced which contains ethylene vinyl acetate, carbon dioxide and other impurities. The gas is not suitable for recycling for further production of copolymer. However, instead of burning the gas as a waste gas, it is desirable to remove the vinyl acetate as well as carbon dioxide from the gas stream and recover the ethylene which may then be used for the production of polyethylene.
One possible method for removal of vinyl esters from a gas stream is to intimately contact the gas with an aqueous basic solution. In the presence of a base, vinyl esters readily hydrolyze to form acetaldehyde and a carboxylate ion. Unfortunately, the acetaldehyde, in the presence of a base, undergoes an aldol condensation in which one molecule of acetaldehyde adds to another molecule to form a 4-carbon hydroxyaldehyde. This hydroxyaldehyde dehydrates to 2-butenal. The butenal can itself undergo condensation reactions and eventually insoluble polymeric materials are formed. These polymers can form in such quantities that the equipment must be periodically shut down for cleaning.
U.S. Pat. No. 3,465,032 discloses a process for removing formaldehyde, acetaldehyde, and butyraldehyde from vinyl acetate. This process involves treating the acetate with sodium bisulfite in order to form the addition products with the aldehyde impurities. The process is conducted at a pH range from about 4.2 to about 6.5. It is also disclosed that when a large excess of bisulfite is present, it reacts with vinyl acetate to form acetic acid and the sodium bisulfite addition product of acetaldehyde.
German Patent application DE 78-2843464 (as abstracted in CA 93:72576k) discloses that aldehydes may be removed from vinyl acetate by treatment with aqueous sodium bisulfite.
French Patent 2,647,782 (as abstracted in Derwent Acc. No. 90-363269/49, and CA 114:82726a) discloses the purification of vinyl chloride containing vinyl acetate by first washing with alkali and then treating the remaining gas with sodium bisulfite at a pH of 6.5 to 8.5. In one example, a stream of vinyl chloride containing 2500 ppm of vinyl acetate was washed with aqueous NaOH. The resultant gas stream contained less than one ppm vinyl acetate and 250 ppm acetaldehyde. This gas stream was then treated with aqueous sodium bisulfite giving a stream of vinyl chloride containing less than 1 ppm vinyl acetate and less than 1 ppm acetaldehyde and 60 ppm SO.sub.2. As set forth more fully below, experiments that we have conducted show that the treatment of vinyl acetate with a caustic leads to the formation of solid polymers. In continuous applications, a buildup of such polymers would be expected in the process of French Patent 2,647,782.
The addition of sodium bisulfite to a carbonyl group is a well-known reaction (see for example, Fundamentals of Organic Chemistry, second edition, T. W. Graham Solomons, pp 621-2, John Wiley & Sons, New York, 1986). The bisulfite addition reaction occurs with aldehydes and with some ketones. The reaction to form the bisulfite addition product is reversible, and often yields a crystalline adduct. Thus, it is useful for separating aldehydes from other materials since the aldehyde can be separated as an adduct and then regenerated by adding either an acid or a base to the adduct. If an acid is used, the aldehyde is released along with SO.sub.2 . In basic solutions, the aldehyde is again liberated along with the production of sulfite ion. Accordingly, sodium bisulfite addition to aldehydes would not be expected to occur in strong basic solutions since the presence of a strong base would tend to decompose any adduct that formed. In addition, a strongly basic solution would drive the equilibrium away from adduct formation by converting bisulfite ions into their conjugate base, i.e., sulfite ions.
In spite of the reversibility of bisulfite addition in basic solution, it is an object of the present invention to provide a process in which a liquid or gaseous process stream containing vinyl esters such as vinyl acetate or aldehyde impurities is treated with sodium sulfite and base in a single step. It is desirable to conduct the treatment in a strongly basic solution so that any vinyl esters, such as vinyl acetate, will be hydrolyzed and removed along with the aldehyde impurities. If the treatment solution is not strongly basic, the vinyl esters are not hydrolyzed rapidly enough to allow their efficient removal, along with aldehyde impurities, in a single step. Such a process is simpler than the two step process of French Patent 2,647,782, and since it is conducted in a highly basic solution, does not leave impurities of SO.sub.2 in the product gas. Furthermore, it is more economical since it requires no more than 0.5 mole of sulfite for each mole of vinyl acetate or aldehyde removed, compared to the 1 or more moles of bisulfite required by prior art processes.